drm/i915/selftests: use live_subtests for live_migrate

Probably a good idea to do an igt_flush_test() at the end of each
subtest, just to be sure the previous work has been flushed and doesn't
somehow interfere with the current subtest.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Chris Wilson <chris.p.wilson@intel.com>
Cc: Andi Shyti <andi.shyti@linux.intel.com>
Cc: Andrzej Hajda <andrzej.hajda@intel.com>
Cc: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221202122844.428006-2-matthew.auld@intel.com